1. Technical Field
The present invention relates to amplifiers and more particularly to an amplifier for use with a telephone receiver.
2. Background Art
Since the advent of the invention of the transistor, many attempts have been made to design telephones replacing the old style voice coil or inductive network with solid state components. In some cases discrete transistors and other components have been used. In other cases integrated circuits have been developed to replace the voice coil network with an equivalent solid state network. Some of the attempts have been successful and others have not. There are several problems encountered. The first of these is a problem with DC power dissipation and DC current supervision of the loop current employed over the telephone line and associated network. Incorrect loop supervision will cause a malfunction of the telephone instrument under certain conditions. For example, the telephone instrument must present less than 300 Ohms DC resistance *to the telephone loop. Secondly, the AC impedance of the instrument should be 600 Ohms. This impedance is difficult to achieve and still provide a high receive gain. A third problem is that the circuit must be stable on all loops in spite of the relatively high impedance nature of the device. A related problem here also is the susceptibility to radio frequency signals with a tendency to pick up and reproduce said signals. A fourth problem involves the audio characteristics which must be within recognized standards. Relative to the receive characteristics this means that the receive level must be at least adequate to prescribe levels over the audio frequency range applicable to telephone communications and yet not too loud. Usually though the levels achieved are not too loud and in some cases they are not loud enough. The present invention is intended to provide loudness that is higher than average but not too loud especially when the application requires extra gain for the received signal. Finally whatever method of providing receiving amplification is used the receiving amplifier must be compatible with some type of transmit amplifier so that the side tone employed can be balanced properly.
Various types of receive amplifier circuits are found in the industry. For example, the GTE NXT telephone manufactured by GTE Communication Systems Corporation uses an operational amplifier technique as do a number of commercially available electronic voice network integrated circuits. Two examples of this are that manufactured by National under their part number TP5700 and that manufactured by Phillips under the part number TER1060-1062 series.
In most of these circuits a phase shifted transmit signal is applied along with the out of phase shifted signal to the receive amplifier input for side tone balance. It is therefore difficult in these circuits to balance the side tone and still have a strong receive amplification factor. In the present invention it has been found that it inherently has side tone cancellation properties due to the differential nature and the method of use employed in the device.